1. Field of the Invention
A light emitting array, and more specifically to light emitting arrays capable of reducing radiation angle of emerging light beams and obviating the generation of light flare, which is suitably adapted to the use in an optical writing unit and an image forming apparatus incorporating the writing unit.
2. Discussion of the Background
In the course of recent developments in downsizing digital image forming apparatuses such as a digital duplication machine, a printer, a digital facsimile and so forth, more attention has been directed to the miniaturization of an optical writing unit in use for digital signal writing.
Current methods for implementing optical writing are broadly divided into two classes. One class is a light scanning method in which a light beam emanated from a light source such as a semiconductor laser or other similar devices is scanned by a light deflecting unit to be formed subsequently as a beam spot through a scan imaging lens incident onto the surface of an image receptor.
The other class is the method utilizing a solid optical writing system, in which light beams, which emerge from the array of light emitting elements such as light-emitting diodes (LEDs) and organic electroluminescent (EL) devices, are shaped as light spots through an imaging element array to be irradiated onto a receptor.
Since the light scanning system utilizes a light deflector for scanning light beams as described above, its optical path length is relatively large to thereby give rise to a drawback of this system.
In contrast, the path length can be made considerably shorter for the solid optical writing system, which offers the key advantage in reducing the size of the writing system. In addition, since any of movable part such as the light deflecting unit is not included in this optical writing system, this also offers a further advantage in forming a quiet system driven with reduced undue noises.
The present invention concerns a light emitting array adapted to the solid optical writing method, an optical writing unit including the light emitting array, and an image forming apparatus incorporating the optical writing unit.
The optical writing unit in use for solid optical writing method will be detailed herein below. In the first place, a known solid optical writing unit is described.
This solid optical writing unit has been formed including at least a light emitting array consisting of a plurality of light emitting elements, and an imaging element array consisting of a plurality of image forming elements. The imaging element array is exemplified by an optical writing unit using a rod lens array as illustrated in FIGS. 22A and 22B.
As to the light emitting array, the array is formed by conventionally using light-emitting diodes (LEDs) as the light emitting elements and by aligning these diodes at predetermined, even intervals.
In addition, the light emitting array consisting of LEDs is formed as shown in FIGS. 22A and 22B by mounting a plurality of LED array chips 324 amounting to approximately from several tens to one hundred on a substrate 323, and the LED array chips 324 are each formed by arranging thereon LEDs amounting to from several tens to one hundred at predetermined intervals.
Furthermore, as shown in FIGS. 22 and 23, the LED array chips 324 are mounted such that LEDs 320 each placed at opposing ends of neighboring array chips 324 have a predetermined distance.
For an LED array for use in the A3 paper width, for example, the resolution of 1200 dpi (dot per inch) can be achieved by arranging sixty LED array chips each mounted with 256 LEDs at the interval of 21.2 μm.
In addition to LEDs described above, organic electroluminescent devices, semiconductor laser elements and other similar devices have been proposed for forming the light emitting array.
In the second place, the configuration in general of an imaging element array to be combined with the light emitting array will be described.
As the imaging element array used in the solid optical writing method, a rod lens array may conventionally be used which are formed of plural rod lenses 332 of the refractive-index distribution type formed as a bundle as illustrated in FIGS. 24A and 24B. The rod lenses 332 are aligned in a double line each piled in staggered position and held altogether between two opposing side plates 334. In addition, an opaque filling member is provided and solidified into the gaps between the rod lenses 332.
As shown in FIG. 21 the rod lens 322 is adapted to form an equal-magnification, erecting image of the light source by adjusting, in a predetermined manner, the distance from the entrance edge plane of the rod lens 322 to the surface of the LED array chip 324 as the light source, and from the exit edge plane of the rod lens to the surface of photoreceptor 326 as an imaging plane.
In addition to the noted rod lens array, a roof prism lens array (RPLA) is known to be used alternatively as an imaging element array, which can be formed integrally with plastics material.
Referring to FIG. 25, the roof prism lens array 120 is formed by including entrance lenses 121 for receiving light beams emanated from the light source, and exit lenses 123 for emanating the beams reflected by a roof prism 122 and for leading to the optical writing unit.
In addition, the roof prism lens array 120 is formed such that a first lens array portion 124 with plural entrance lenses 121 aligned thereon and a second lens array portion 125 with plural of the exit lenses 123 aligned thereon are configured to be perpendicular with one another, and that a roof prism portion 126 with the plural roof prism lenses 122 aligned thereon in the same aligning direction is configured between the entrance lenses 121 aligned on the first lens array portion 124 and the exit lenses 123 aligned on the second lens array portion 125.
Furthermore, the center of any one of the entrance lenses 121 provided on the first lens array portion 124, the ridge line portion 127 corresponding to the abovementioned entrance lens 121 of the roof prism portion 126, and the center of the exit lens 123 provided on the second lens array portion 125 are altogether configured to be in the direction perpendicular to that of the alignment of the abovementioned entrance lenses 121 and exit lenses 123.
A light shield material may be provided further between the lenses to prevent the transmission of undue light beams such as stray light, for example.
Light beams incident on the face of the entrance lens 121 are deflected approximately by a right angle by the roof prism 122 and subsequently emanated from the face of exit lens 123. By interposing the roof prism lens array 120 between the light source and the imaging surface, therefore, the imaging of the light source can be achieved on a prescribed surface of the image bearing member such as, for example, photoreceptor drum.
Incidentally, the noted optical systems with either the rod lens or the roof prism lens are both optical systems for forming equal-magnification, erecting images, which are adapted to receive light beams emanated from one light source with plural lens faces, emanate from further faces each provided on the side opposing to the receiving faces, and generate the image as a focused spot on an imaging surface.
For the optical writing unit in use for solid optical writing method using imaging element array incorporating the rod lens or the roof prism lens, it is generally known the light emanated from light emitting elements can be transmitted onto the photoreceptor, as an image bearing member, in the proportion of merely few percent (about from 1 to 5 percent).
Although it is necessary to transmit a predetermined amount of light (or light intensity) to properly carry out light exposure onto the surface of photoreceptor, as the image bearing member, the light intensity generally comes to decrease during the passage through imaging element array.
Therefore, it is desirable to increase the intensity of light transmitted to the photoreceptor, and accordingly this may be achieved by increasing the light intensity itself by means of the increase in current input to the light emitting elements, for example.
The noted means for increasing current input, however, results in a concomitant increase of power consumption. Therefore, it is desirable to increase the intensity of light transmitted to the photoreceptor without increasing the intensity itself of light emission from the light emitting elements. This necessitates increasing light transmission efficiency of optical systems in use for the optical image writing.
Several means may be cited for increasing light transmission efficiency such as
(1) using a lighter (or less dark) imaging element array,
(2) in place of the imaging element array, utilizing microlenses for converging light beams emanated from the light emitting elements directly on the photoreceptor as a light spot, and/or
(3) appending an optical device capable of improving light transmission efficiency to the existing imaging element array.
As to the noted means (1), for example, there may be provided the improvement in opening ratio of the rod lens array. This is, however, considered difficult at present since a rod lens array is hard to be formed so as to attain light spot characteristics uniform enough over receptor area for obtaining satisfactory pictorial images while maintaining the opening ratio improvement, as well.
In the means (2) mentioned above, the method has been disclosed in which plural microlenses and waveguide structures are formed both being brought into one-to-one correspondence with light emitting elements. As a result, light spots are formed on a photoreceptor without intervening imaging element array (for example, in Japanese Laid-Open Patent Applications No. 9-187991 and 10-181089).
In this method, however, since the optical system for imaging is formed primarily with the microlenses, the characteristics of light spots on the photoreceptor are considerably influenced by the form accuracy of the microlenses after assuming light emitting diodes are produced with sufficient uniformity of light emission.
Thus, there gives rise to a drawback of this method in which the variation of the shape of microlenses manifests itself as that of the characteristics of light spots on the photoreceptor, and this may prevent the formation of satisfactory picture images.
Although it is thus necessary to form uniform microlenses to form excellent images, such images are difficult to be acquired under the present conditions. In addition, it is also difficult the light beams emanated from light emitting diodes are all led to form spots on the photoreceptor (the proportion of 90 percent level is cited in the application '991). The rest of the beams are still emitted from LEDs toward the photoreceptor only resulting in undue light flare onto the photoreceptor.
It is disclosed in the application '991 that light beams emanated from light emitting elements are led by way of light guides and focused by lenses to form light spots on the photoreceptor. Even though the generation of the light flare is decreased to a certain extent by the light guides, the focusing characteristics is considerably influenced by the form accuracy of the lens provided in the light guide structure.
As a result, there gives rise to a drawback in which the variation of the shape of microlenses reveals itself as that of the characteristics of light spots on the photoreceptor, and this may prevent the formation of satisfactory picture images.
In addition, a further drawback is alluded to in that process steps required for forming the light guide structure are complicated because the light guides are formed by etching and their lens portions have to be fabricated through photolithography and etching process steps.
As to the means (3) noted above, an improvement has been disclosed in light transmission efficiency of optical system, which is attained by combining a lens array capable of narrowing the radiant angle of light emitting elements with the existing imaging element array (for example, the Japanese Laid-Open Patent Application No. 11-170605).
In the method described in the application '605, however, difficulties may arise. Namely, since the lens array for narrowing the radiant angle of light emitting elements is fabricated on the surface of a transparent substrate having a certain thickness, light emerging from the next and/or second next lens may result in light flare because of the substrate thickness even the surface areas between the neighboring lenses are adapted beforehand to shield light. This may unduly affect the transmission efficiency.
In addition, the light reflected by the surface of the lenses and light shielding layers may propagate through, and discharged from the transparent substrate. This may also be one of the causes for generating light flare. FIG. 26 is prepared for illustrating the modes of transmission of the light flare.
Besides the abovementioned means for improving light transmission efficiency of optical system, a method has been disclosed (in Japanese Laid-Open Patent Application No. 2002-164579, for example) of increasing the intensity of light emanated from light emitting elements without increasing current input, in which a microlens is provided to each of the light emitting elements and which the existing optical imaging system is utilized as it is.
According to this method, since the optical capability is retained as that of the existing system and the microlens has little imaging function by itself, several difficulties can be alleviated such as undue increase of, and relatively easy increase in variation of spot size on imaging surface.
In the method described above in the application '579, however, since the microlens is provided directly on a light emitting plane of the light emitting element, it is difficult to make the apex height for the microlens larger than its curvature radius, and accordingly to reduce the radiation angle of the light beams emanated from the light emitting element.
In addition, since the shape of the microlens is considerably dependent on the gradient of the base of light emitting element, possible variation of the shape over respective microlenses may cause to the fluctuation of light intensity of the spots.
Still in addition, a further method has been disclosed (in Japanese Laid-Open Patent Application No. 2001-119072, for example) of reducing the radiation angle without using microlenses. Namely, cylindrical micro mirrors are provided to encapsulate each of light emitting illuminants such that light beams emerging from the element having a relatively large radiation angle are reflected by the micro mirror to be confined within a reduced radiation angle.
This method described in the application '072 appears effective in reducing the radiation angle of light beams having a relatively large radiation angle.
However, in order to reduce with this method the radiation angle of light beams emerging in larger angles such as approximately from 30° to 40°, the height of the micro mirrors has to be increased. This may result in drawbacks such as difficulty in forming micro mirrors having proper thickness and/or unduly enlarged light spot caused by light reflected by mirror surfaces and the method does not appear favorable in practice.